1. Field of the Invention
This invention relates to xerographic imaging, and, more particularly, to transparencies that are imageable by means of a xerographic process.
2. Discussion of the Prior Art
As is well known, xerographic imaging commonly involves imparting a uniform electrostatic charge, either positive or negative, depending on the specific machine under consideration, to a photoconducting surface which will hold a charge only in the dark, such as a selenium coated drum. This may be accomplished by passing the drum under a series of corona-discharge wires in the dark. The photoconducting surface is then exposed through a lens system to a document or article bearing the image which is to be formed. In areas where light strikes the photoconducting surface the charge is dissipated and flows off through a conducting support to ground, with electrostatic charge remaining largely intact in the image areas. Next, oppositely charged particles, called "toner", comprising a colored thermoplastic resin is brought into contact with the photoconducting surface, where it clings by electrostatic attraction to the charged areas of the surface, forming a visible image called a "powder image". A sheet which is to receive the image is placed over the powder image, and is given a charge, such as by use of corona-discharge wires. As a result, a large portion of the charged powder on the photoconducting surface is transferred to the sheet. Finally, the toner is fused to the sheet by application of heat, pressure, or a combination of both. At present, xerographic copies which use plain paper for reproduction of images from a master generally employ a dry powder ("toner") to form the image.
The toner powder is generally comprised of a polymeric resin, dyes or pigments, and various additives to control the surface charge and other characteristics necessary for reliable operation. The most common method of fusing the toner to the copy sheet is to heat it, either with lamps or heated rolls. Heated rolls combine heat and pressure, thus reducing the amount of heat needed. Cold pressure fusing, which uses no heat and very high pressure, may also be used.
The principles of image formation by the application of toner to a medium in a xerographic imaging machine do not depend upon whether that medium is film or paper. In practice, however, it has been found that xerographic imaging onto film presents difficulties that imaging onto paper does not.
First, film is smoother and less electrically conductive than paper. As a result, film sheets have a much greater tendency to cling to like sheets than do paper sheets. When large numbers of sheets of film are stacked, they tend to cling to one another, forming a solid block. This phenomenon, called "blocking", prevents many types of film from being reliably fed from conventional paper trays using the automatic feeding mechanisms of the type found in most xerographic imaging machines
There are several ways of modifying film in order to reduce blocking. One such method involves roughening the surface, so as to reduce the intimacy of contact between adjacent sheets. This can be done either by adding particulate matter to the film itself, or by adding the particulate matter as a coating, using a polymeric material as a binder to hold the particulate matter in place. However, surface roughening has a disadvantage in that roughened surfaces tend to scatter transmitted light, thus reducing the image contrast when such films are viewed in the transmission mode, as in overhead projection.
Another method for reducing blocking and thereby improving film feeding is the addition of antistatic materials, such as quaternary ammonium salts, to a coating of the film. This serves to reduce electrostatic cling under certain circumstances, and may also reduce the coefficient of friction between the film surface and the surface of other film sheets.
Another method for reducing electrostatic cling is to attach a sheet of paper of the about the same size as the film sheet to the film sheet itself, typically by applying a strip of adhesive along one edge of the film sheet, and then attaching the paper sheet to the film sheet. Although effective in reducing blocking, the attached paper sheet must be removed before the imaged film sheet can be shown by an overhead projector. An imageable film sheet that has been constructed in this manner is disclosed in U.S. Pat. No. 3,618,752.
A second problem that has been encountered in substituting film sheets for paper sheets in xerographic imaging machines is that some machines of this type employ an optical sensor to determine the presence of paper sheets. Because these sensors depend upon the opacity of paper to be effective, they often cannot operate effectively when a transparent film sheet is used. In order to overcome this problem, it has become standard practice to print an opaque strip along one edge of each sheet of film. However, these strips are unacceptable to some users because the projected image area is reduced from 81/2.times.11 inches down to as low as about 77/8.times.11 inches.
Another method of modifying film sheets to operate effectively in machines having optical sensors is the application thereto of a removable opaque stripe in place of the printed opaque stripe described previously. This sheet is typically a strip of paper or tape, applied at or near one edge of the sheet of film, by means of an adhesive which allows removal of the strip without damage to the film and without leaving a visible adhesive residue on the film. Unlike the coextensive sheet described previously, the strip can be left on the film during viewing, but it can be removed in cases where this proves objectionable. Further, paper or tape strips have greater opacity than do printed strips, thereby improving the reliability with which the optical sensor responds to the sheet.
The use of a single strip along one edge of the film sheet for the purpose of activating optical sensors has the disadvantage that the operator must load the film sheets into the paper tray in such a way that the edge bearing the strip enters the machine with the proper orientation, because the strip-bearing edge must pass a particular location to activate the optical sensor. A method for eliminating this disadvantage has been disclosed in U.S. Pat. No. 4,637,974, wherein a strip is applied along each of the four edges of the sheet, so that no matter which way the sheets are placed in the machine, there will always be an opaque area on the edge required by the optical sensor. This film sheet has the disadvantage that the edge strips intrude into the transparent area not just along one edge, but along all four edges, thereby reducing the viewable area to a greater degree.
Another aspect wherein xerographic imaging onto film differs from xerographic imaging onto paper is in the method by which the image is viewed. The most common reason for using film rather than paper as the imaging medium is to enable the image to be viewed by light transmitted through the image bearing medium, as in overhead projection. A problem which commonly arises in showing images carried on standard size sheets of film by overhead projection is that the stage aperture of the projector is typically larger than the sheet of film. As a result, the image of the aperture appearing on the screen will be larger than the image of the sheet of film. Those areas of the projector stage not covered by film will therefore appear as very bright areas on the screen. These bright areas tend to make the image look dark and of poor contrast, and are therefore objectionable to many viewers.
A common method of eliminating such bright areas is to attach the sheet of film to a cardboard frame having outside dimensions of sufficient size to cover the projector stage, and inside dimensions which result in a pleasing screen appearance. Such frames are commercially available. They have the disadvantages of being bulky, being difficult to transport and store, and requiring considerable labor in attaching the imaged sheets to them.
The work of attaching the imaged sheets of film to frames can be eliminated by the use of preframed transparencies, which are also commercially available. Preframed transparency films are constructed by applying opaque borders to sheets of film which are of sufficiently large size to cover an overhead projector stage. It has been found, however, that such large sheets of film are difficult to feed reliably from many of the paper supply trays found in conventional xerographic imaging machines.
Transparent film sheets having removable strips suffer from disadvantages which tend to outweigh their advantages. The high feeding reliability is offset by the limitation of having to accept either reduced format or the added work of removing the strip. If a framed image is desired, additional work is required to apply such frames. Preframed transparency films exhibit great difficulty in feeding.